----- Original Message -----

From: John S

To: TekScopes@...

Sent: Wednesday, June 19, 2013 3:01 PM

Subject: [TekScopes] CRT magnetic screens: Mumetal?

 

I've always assumed that the blue screen around the CRT on Tek scopes (400 series etc) was made of Mumetal. Is this actually true?

John

From: Steve
Subject: [TekScopes] Re: CRT magnetic screens: Mumetal?
To: TekScopes@...
Date: Wednesday, June 19, 2013, 10:05 PM

 

--- In TekScopes@..., "John S" wrote:
>
>
> I've always assumed that the blue screen around the CRT on Tek scopes (400 series etc) was made of Mumetal. Is this actually true?
>
> John
>

Yes.

Tek fabricated them from raw stock. After they were cut, bent and spot welded, they went into an oven to be annealed (mu metal loses its shielding properties if physically bent or modified (drilling holes etc.) after annealing. If you drop a shield on the floor and the corner bends - throw it away.) The ovens operated at high temperature in a pure hydrogen (reducing) atmosphere. Opening an oven door before it had cooled posed an explosion hazard. To minimize the danger, Tek used several small ovens, each could only contain about 4 or 6 shields.

- Steve

Thanks for the confirmation guys: must have been an expensive component. There's no "warning" lable on either the shield or the manual to tell you not to knock etc.

John

While I've never played with Mu-metal, I have kept all my shields too, knowing about the properties of the material. My question- I know bending and heating affect the metal, but I didn't know that cutting it had an effect. What is the best way to cut it? I'd assume no deflection, so a small cutting disc on well supported stock with coolant would seem to be the best approach?


-Dave

----- Original Message -----
From: "Ed Breya" <edbreya@...>
To: TekScopes@...
Sent: Wednesday, June 19, 2013 4:36:41 PM
Subject: [TekScopes] Re: CRT magnetic screens: Mumetal?






I think in the earlier days, or in some manuals, there were warnings about avoiding stresses on the material. It's not practical to re-anneal to original performance if seriously banged up, but the overall shielding is still pretty good even so, and certainly better than nothing. For our purposes, I wouldn't worry too much about a few dings here and there - just don't put any magnets near the defects, which could become slightly magnetized and possibly cause distortion in the CRT.

I've saved the shields from every CRT item I've ever junked out, and have worked and re-used the material for a number of applications. It loses some of its effectiveness near any cuts or sharp bends, but large planar areas that aren't stressed too much tend to be OK.

Ed

--- In TekScopes@... , "John S" <John@> wrote:

Thanks for the confirmation guys: must have been an expensive component. There's no "warning" lable on either the shield or the manual to tell you not to knock etc.

John

Hi Steve,

Can you save a dropped shield with a AC degauss coil?  How high a temperature
was needed to anneal the Mu metal?
A local guy over here is sitting on a little stock of Mu, maybe I'll get it for future
use! you never know!

see ya

Peter
 
 




________________________________
From: Steve <ditter2@...>
To: TekScopes@...
Sent: Wednesday, June 19, 2013 5:05:53 PM
Subject: [TekScopes] Re: CRT magnetic screens: Mumetal?



 
--- In TekScopes@..., "John S" <John@> wrote:

I've always assumed that the blue screen around the CRT on Tek scopes (400 series etc) was made of Mumetal. Is this actually true?

John

Yes.

Tek fabricated them from raw stock. After they were cut, bent and spot welded, they went into an oven to be annealed (mu metal loses its shielding properties if physically bent or modified (drilling holes etc.) after annealing. If you drop a shield on the floor and the corner bends - throw it away.) The ovens operated at high temperature in a pure hydrogen (reducing) atmosphere. Opening an oven door before it had cooled posed an explosion hazard. To minimize the danger, Tek used several small ovens, each could only contain about 4 or 6 shields.

- Steve

Hi Ed

Thanks for the comments.  I'll see if the stock is annealed.  I have this kiln I hooked up to
Digital temp control and SSR, I flood the chamber with CO2 gas as I heat small steel
parts for heat treating, then quench in oil, works better then using a torch.

Once the kiln is hot I could switch my gas from CO2 to H2 when all the O2 is purged. 

Q >  Is it possible to get a reducing gas atmosphere without using H2 or the same affect
with another gas, like above?

(I like looking for ways to get 'factory' results in a home workshop!...looking for an excuse
to do die attach and wire bonded hybrid circuits here!)

see ya,

Pete




________________________________
From: Ed Breya <edbreya@...>
To: TekScopes@...
Sent: Thursday, June 20, 2013 10:38:53 AM
Subject: [TekScopes] Re: CRT magnetic screens: Mumetal?



 
It's not about degaussing, it's a problem with losing the grain orientation that was imparted during annealing.

Annealing it is very difficult since the temperature is quite high - like approaching its melting point, and the hydrogen atmosphere is very risky. It occured to me once to maybe try an oxy-hydrogen torch with an over-rich flame, but then the problem is what do you do about the opposite side, and the transition region from flame to not, and hot to cold. It really needs to be in a uniform environment.

If you buy raw sheet stock, it may be un-annealed, so won't work as well as that in a finished piece that definitely would have been. If the stock is labeled so or known to be annealed, then it's good stuff to keep around.

Ed

--- In TekScopes@..., Peter Hildebrandt <petertech99h@> wrote:

Hi Steve,

Can you save a dropped shield with a AC degauss coil?  How high a temperature
was needed to anneal the Mu metal?
A local guy over here is sitting on a little stock of Mu, maybe I'll get it for future
use! you never know!

see ya

Peter
 
 




________________________________
From: Steve <ditter2@>
To: TekScopes@...
Sent: Wednesday, June 19, 2013 5:05:53 PM
Subject: [TekScopes] Re: CRT magnetic screens: Mumetal?



 
--- In TekScopes@..., "John S" <John@> wrote:

I've always assumed that the blue screen around the CRT on Tek scopes (400 series etc) was made of Mumetal. Is this actually true?

John

Yes.

Tek fabricated them from raw stock. After they were cut, bent and spot welded, they went into an oven to be annealed (mu metal loses its shielding properties if physically bent or modified (drilling holes etc.) after annealing. If you drop a shield on the floor and the corner bends - throw it away.) The ovens operated at high temperature in a pure hydrogen (reducing) atmosphere. Opening an oven door before it had cooled posed an explosion hazard. To minimize the danger, Tek used several small ovens, each could only contain about 4 or 6 shields.

- Steve

----- Original Message -----

From: Steve

To: TekScopes@...

Sent: Friday, June 21, 2013 5:18 PM

Subject: [TekScopes] Re: CRT magnetic screens: Mumetal?

 

--- In TekScopes@..., "Ed Breya" wrote:
>
> If you flood it first with CO2, I think the mu-metal will tend to react with it at the high temperature, making it worse. Same with air. C and O are supposed to be removed from the alloy by the H2 reducing atmosphere during annealing, so you would probably need it throughout the process. I'm pretty sure also that H2 will readily ignite at 2,000 deg F or so, so any air incursion will likely have immediate effects, although the density of the gas will be fairly low. Since you'd have to keep a positive internal pressure, the hot H2 may ignite on the way out of the leakage paths, so you could have a bunch of tiny torch flames all around the outside.
>
> I wouldn't want to be anywhere near it during the cookout. I have a small Paragon kiln, and I wouldn't want to lift the lid even a tiny crack when it's up to temperature - imagine the top of your kiln popping off on its own, and maybe glowing chunks of firebrick and heater element flying around.
>
> Before attempting anything like that, you should study up on how it's done in industry, especially the temperature range and how long it takes, and appropriate safety precautions.
>
> Ed

CO2 is not safe for this purpose. At high temperatures, it disassociates into carbon and oxygen. A classic high school chemistry demonstration is to ignite a small strip of magnesium metal and drop it into an open beaker filled with CO2. (carbon dioxide is much denser than air, so a filled beaker will remain filled with the top open, barring any strong drafts.) The magnesium will go from gentle burning to a violent flash flame, on the verge of exploding.

If the oven were heated to 2000 degree F with a pure CO2 atmosphere and then flooded with H2, it could explode. I believe the process Tek used flooded the oven with H2 while it was still cool, then turned on the heat.

- Steve

To add to Steve's CO2 comments, if you place a strip of Mg ribbon between two blocks of dry ice and light the exposed end, it will burn right into the blocks until the Mg is consumed. The blocks glow at a very high intensity.

Also, you need to be aware of the effects of the H2 atmosphere on the oven and its components.

Regards,
Tom

----- Original Message -----
From: Steve
To: TekScopes@...
Sent: Friday, June 21, 2013 5:18 PM
Subject: [TekScopes] Re: CRT magnetic screens: Mumetal?



--- In TekScopes@..., "Ed Breya" <edbreya@> wrote:
>
> If you flood it first with CO2, I think the mu-metal will tend to react with it at the high temperature, making it worse. Same with air. C and O are supposed to be removed from the alloy by the H2 reducing atmosphere during annealing, so you would probably need it throughout the process. I'm pretty sure also that H2 will readily ignite at 2,000 deg F or so, so any air incursion will likely have immediate effects, although the density of the gas will be fairly low. Since you'd have to keep a positive internal pressure, the hot H2 may ignite on the way out of the leakage paths, so you could have a bunch of tiny torch flames all around the outside.
>
> I wouldn't want to be anywhere near it during the cookout. I have a small Paragon kiln, and I wouldn't want to lift the lid even a tiny crack when it's up to temperature - imagine the top of your kiln popping off on its own, and maybe glowing chunks of firebrick and heater element flying around.
>
> Before attempting anything like that, you should study up on how it's done in industry, especially the temperature range and how long it takes, and appropriate safety precautions.
>
> Ed

CO2 is not safe for this purpose. At high temperatures, it disassociates into carbon and oxygen. A classic high school chemistry demonstration is to ignite a small strip of magnesium metal and drop it into an open beaker filled with CO2. (carbon dioxide is much denser than air, so a filled beaker will remain filled with the top open, barring any strong drafts.) The magnesium will go from gentle burning to a violent flash flame, on the verge of exploding.

If the oven were heated to 2000 degree F with a pure CO2 atmosphere and then flooded with H2, it could explode. I believe the process Tek used flooded the oven with H2 while it was still cool, then turned on the heat.

- Steve